Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection and metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield.
Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures. To perform high throughput measurements of modern semiconductor structures, including high aspect ratio structures, a wide range of illumination wavelengths must be employed, ranging from vacuum ultraviolet (VUV) wavelengths through infrared (IR) wavelengths.
Similarly, when inspecting specular or quasi-specular surfaces such as semiconductor wafers bright field (BF) and dark field (DF) modalities may be used, both to perform patterned wafer inspection and defect review. In BF inspection systems, collection optics are positioned such that the collection optics capture a substantial portion of the light specularly reflected by the surface under inspection. In DF inspection systems, the collection optics are positioned out of the path of the specularly reflected light such that the collection optics capture light scattered by objects on the surface being inspected such as microcircuit patterns or contaminants on the surfaces of wafers. Viable inspection systems, particularly BF inspection systems, require high radiance illumination and a high numerical aperture (NA) to maximize the defect sensitivity of the system.
Current wafer inspection and metrology systems typically employ a broad range of illumination sources including VUV sources, such as laser sustained plasmas. Laser sustained plasmas are produced in high pressure bulbs surrounded by a working gas at lower temperature than the laser plasma. Substantial radiance improvements are obtained with laser sustained plasmas. Atomic and ionic emission in these plasmas generates wavelengths in all spectral regions, including shorter than 200 nm when using either continuous wavelength or pulsed pump sources. Excimer emission can also be arranged in laser sustained plasmas for wavelength emission at 171 nm (e.g., xenon excimer emission). Hence, a simple gas mixture in a high pressure bulb is able to sustain wavelength coverage at deep ultraviolet (DUV) wavelengths with sufficient radiance and average power to support high throughput, high resolution BF wafer inspection, for example.
The availability of high power illumination sources places a significant burden on the optical components employed to collect and focus the high power radiation onto the specimen under measurement. Contamination and absorption issues can lead to failure of optical components. Monitoring the thermal characteristics of optical components in modern metrology and inspection systems becomes critical to ensure tool performance and reliability.
In some examples, temperature is measured by sensors (e.g., a thermocouple) placed in mechanical contact with an optical component. Contact temperature measurements are simple to implement in some situations, but contact thermal measurements have a number of significant limitations. For example, mechanically attaching a sensor directly to an optical element can be very difficult and can potentially damage the optic itself. Even if attached to the optical element, it may show incorrect temperature due to poor thermal conductivity of the optical element. The thermocouple wire itself has relatively high thermal conductivity and may change the temperature of the contact point. In addition, the contact sensor can absorb and scatter light in the system. Thus, the physical presence of the contact sensor changes the temperature at the point of contact and the temperature of the optical system as well. In many situations, installation of the contact sensors is impossible inside sensitive optical systems due to cleanliness requirements, optical alignment sensitivity, etc. These problems manifest themselves in VUV systems, among others. Contact thermal measurements only measure the temperature at the point of contact. Often, this is not the optical component itself but an optical mount with a very different temperature from the optical component of interest. Typically, temperature measurements at one or two points at the periphery of an optical element do not provide enough information to recreate the complete temperature distribution of interest. Generally, an accurate estimate of the total temperature distribution or the peak temperature at the center of optic field is preferred.
In some other examples, temperature is measured by thermal imaging. Thermal imaging does not require locating sensors onto the optical element. Thus, in many applications, thermal imaging of optical components is preferred. Typically, thermal imaging based temperature measurements are performed by imaging a target object with one or more calibrated IR cameras. However, thermal imaging based temperature measurements also suffer from significant limitations. For example, thermal cameras are expensive and require significant integration effort to achieve accurate results. In addition, thermal based temperature measurements require an unobscured view of the measured object, and this is difficult or impossible to achieve in many optical systems. Also, in many cases, the optical components themselves either transmit or reflect IR, and for that reason they are invisible to the IR cameras. This is the case for VUV optical components that include VUV transparent materials such as Magnesium Fluoride, Calcium Fluoride, and Lithium Fluoride, and VUV reflective materials such as metal mirrors.
In summary, ongoing reductions in feature size and increasing depths of structural features impose difficult requirements on optical metrology systems. Optical metrology systems must meet high precision and accuracy requirements for increasingly complex targets at high throughput to remain cost effective. In this context, high power optical systems must be employed and it becomes increasingly important to monitor the thermal characteristics of optical components during operation. In many examples, it has not been possible to perform these measurements. Thus, improved systems and methods to overcome these limitations are desired.